Different operating conditions are known in conjunction with direct petrol injection systems. The common factor is that fuel is injected under high pressure directly into a combustion chamber. The mixture is then formed within the combustion chamber. Conventionally a distinction is made between the homogeneous and lean operating modes. In homogenous operation a mixture is present which is distributed homogeneously over the entire combustion chamber. In stratified or lean injection operation there is only a mixture with a excess air in factor the area of the spark plug λ≦1. The remaining volume of the combustion chamber is filled with induced fresh air, an inert gas from the exhaust gas recirculation or a very lean fuel-air mixture, so that overall an excess air factor of λ≦1 is produced.
In addition to these conventional operating modes, a further operating mode is increasingly being seen as promising, which is similar to the operation of the self-ignition diesel engine. This is known as HCCI (Homogeneous Charge Compression Ignition) operation and represents an auto-ignition combustion process, in which the time of ignition and thereby the sequence of combustion is controlled via the reactive quantity of energy in the cylinder. To provide a sufficient energy level use is usually made of exhaust gas recirculation via external setting means within the framework of exhaust gas recirculation or by a suitable gas exchange valve control within the framework of an internal exhaust gas recirculation.
For setting of the temperature level and thereby the energy level in the combustion chamber via the exhaust gas recirculation rate however it is necessary to take into account that this can only take place within specific limits. Since the exhaust gas recirculation rate influences not only the temperature level in the combustion chamber but also the mixture ratio of air, fuel and exhaust gas, it is under some circumstances not possible to select an exhaust gas recirculation rate which is optimum both with regard to the temperature in the combustion chamber and with regard to the said air-fuel mixture ratio. Thus compromises can be necessary when setting the exhaust gas recirculation rate to ensure reliable operation of the internal combustion engine.
In the context of conventionally ignited internal combustion engines it has already been proposed that a cooled exhaust gas recirculation be used, whereby this cooling of the exhaust gas is aimed especially at reducing the nitric oxide emissions. In this context reference is made for example to the German periodical MTZ Motortechnische Zeitschrift 60 (1999) 7/8, page 470 ff.: “Einhaltung zukünftiger Emissionsvorschriften durch gekühlte Abgasrückführung” (complying with future emission regulations using cooled exhaust gas recirculation) by Karl-Heinrich Losing and Rainer Lutz.